Conventionally, a motor drive device including a function for detecting insulation deterioration of a motor winding (coil) by applying a voltage charged across a smoothing capacitor of a DC link unit between the motor winding and the ground is known (e.g., Japanese Patent Publication No. JP-B-4554501). In a conventional motor drive device, insulation deterioration of a motor is detected by measuring a leakage current flowing between a motor coil and the ground by applying a voltage charged across a smoothing capacitor of a direct-current power source (DC link unit) connected to an inverter between the motor coil and the ground after shutting off an alternating-current power source by a switch.
Further, a motor drive device including a plurality of inverter units for driving a plurality of motors is known, which calculates an insulation resistance of each motor by detecting voltages and currents at a time at the same timing by using one voltage detection unit of a common converter unit and a plurality of current detection units of each inverter unit for each motor (e.g., Japanese Patent Publication No. JP-B-4565036. Hereinafter, referred to as “Patent Literature 2”).
Each of the above-mentioned conventional techniques makes use of a high voltage charged across a smoothing capacitor originally included in the inverter as a power source for measurement. Therefore, it is not necessary to separately provide a dedicated power source for measurement, and therefore, the configuration is simple and each technique is an excellent method in that measurement results with a high accuracy are obtained because a high measurement voltage is obtained.
In order to measure a high insulation resistance value with a high accuracy, it is advantageous to increase a measurement current value by increasing a voltage that is applied. This is obvious from the fact that many measuring instruments for measuring an insulation resistance, which is called an insulation resistance meter or a megohm tester, set high measurement voltages, such as 250 [V], 500 [V], and 1,000 [V].
FIG. 1 illustrates an example of a configuration of a motor drive device that uses the conventional technique disclosed in Patent Literature 2.
The measurement procedure of the insulation resistance of a motor that makes use of a conventional motor drive device 1000 is as follows. First, an alternating-current power source 1002 is disconnected from a rectification circuit 1003 by turning off a first switch 1001 in a state where all semiconductor switching elements 1051 to 1056 of an inverter 1005 including the semiconductor switching elements 1051 to 1056 and diodes 1051d to 1056d connected in inversely parallel thereto are turned off. Next, a second switch 1009 and a third switch 1010 are turned on and a plus side terminal 1042 of a smoothing capacitor 1041 of a DC link unit 1004 is connected to the ground. As a result of that, a charged voltage of the capacitor 1041 of the DC link unit 1004 is applied between coils 1061 to 1063 of a motor 1006 and the ground. At this time, a current flowing through a closed circuit indicated by a dotted line (see FIG. 1) formed by the capacitor 1041, the motor coil (e.g., 1062), and the ground is measured by a current measurement circuit 1007 provided between the motor coil 1062 and a minus side terminal 1043 of the capacitor 1041 of the DC link unit 1004. At the same time as this, a voltage between terminals of the capacitor 1041 of the DC link unit 1004 at this time is also measured by a voltage measurement circuit 1008 connected in parallel to the DC link unit 1004 by using a detection resistor 1081 and a voltage division resistor 1082. Then, the insulation resistance value between the motor 1006 and the ground is found from the voltage value and the current value obtained by the above measurement.
FIG. 2 illustrates an equivalent circuit representing the relationship of connection between the closed circuit and the semiconductor switching element at the time of measurement of the insulation resistance in the configuration in FIG. 1 in relation to the conventional motor drive device. At the time of measurement, the first switch 1001 is in the OFF state, and therefore, the alternating-current power source 1002 is disconnected. Further, the second switch 1009 and the third switch 1010 are in the ON state, and therefore, the plus side terminal 1042 of the DC link unit 1004 is connected to the ground and the current measurement circuit 1007 is connected to the minus side terminal 1043 of the DC link unit 1004. “RU−IGBT” represents an equivalent insulation resistance value when the semiconductor switching elements 1051, 1053, and 1055 of an upper arm of the inverter are OFF, “RD−IGBT” represents an equivalent insulation resistance value when the semiconductor switching elements 1052, 1054, and 1056 of a lower arm of the inverter are OFF, “Rm” represents an insulation resistance value between the coil of the motor to be measured and the ground, and “RC” represents a resistance value when the serial connection of a voltage division resistor 1072 and a current detection resistor 1071 of the current measurement circuit 1007 is represented by one resistor, respectively.
In the conventional technique, due to high voltage charged across the smoothing capacitor 1041 a leakage current that flows through the semiconductor switching elements 1051 to 1056 in the OFF state of the inverter 1005 occurs and these currents overlap the measurement current, and therefore, there has been a problem that the measurement accuracy is reduced at high temperatures particularly when the leakage current flowing through the semiconductor switching elements increases.
In the above description, the “leakage current flowing through the semiconductor switching elements in the OFF state” refers to, in the example of an IGBT, a leakage current that flows from the collector to emitter in the state where the IGBT is OFF.
The leakage current in the OFF state is specified as the electrical characteristics represented by a symbol ICES in the IGBT and is called a “collector-emitter leakage current”. The collector-emitter leakage current (ICES) is specified as a leakage current that flows from the collector to emitter when a specified voltage (usually, a maximum rated voltage) is applied between the collector and emitter in the state where the gate and emitter are short-circuited, i.e., in the state where the IGBT is perfectly turned off.
The collector-emitter leakage current (ICES) of the IGBT has strong temperature dependence and the leakage current ICES has the characteristics that the leakage current ICES increases exponentially when temperature rises.
Further, it is known that such characteristics that the leakage current in the OFF state increases as temperature rises are observed not only in the IGBT but also in other semiconductor switching elements, such as MOS-FET. For example, in the case of the MOS-FET, the characteristics are specified as the electrical characteristics represented by a symbol IDSS as the drain-source leakage current in the OFF state.
In general, an increase in the leakage current ICES at high temperatures in the IGBT for the use as an inverter for driving a motor is regarded as a problem mainly from a viewpoint of an increase in loss. However, even if the leakage current ICES is on the order of several ten [μA], which does not result in any problem from the viewpoint of a loss in a motor drive device, the leakage current ICES will cause a reduction in the measurement accuracy in the insulation resistance measurement of a motor in the conventional technique.
Specifically, as is obvious from FIG. 2, the problematic point of the conventional technique is that part of the leakage current flowing through the semiconductor switching elements 1051 to 1056 in the OFF state overlaps the current (see a current I indicated by a dotted line arrow in FIG. 2), which is the original target of measurement, flowing through the insulation resistor Rm between the motor 1006 and the ground (see FIG. 1), and flows directly into the current measurement circuit 1007 (see ILEAK indicated by an alternate long and short dash line in FIG. 2), and therefore, the leakage current flowing through the semiconductor switching elements 1051 to 1056 directly causes a measurement error.
In the conventional technique also, if the current flowing through the semiconductor switching elements 1051 to 1056 is so sufficiently small that it can be ignored compared to the measurement current, it is unlikely that the measurement accuracy of the insulation resistance measurement of the motor 1006 reduces to bring about a practical problem.
A criterion to determine whether or not the equivalent insulation resistance value of the semiconductor switching elements 1051 to 1056 in the OFF state affects the measurement accuracy of the insulation resistance measurement of the motor can be considered as follows. If the equivalent insulation resistance value of the semiconductor switching elements 1051 to 1056 in the OFF state is sufficiently large compared to the insulation resistance value of the motor 1006 to be measured, it can be considered that no problematic influence will occur. However, in the case where the equivalent insulation resistance value of the semiconductor switching elements 1051 to 1056 is equal to or less than the insulation resistance value of the motor 1006 to be measured, it will be difficult to perform insulation resistance measurement with substantially high accuracy. This is also obvious from the equivalent circuit in FIG. 2.
FIG. 3 is a graph indicating the relationship (temperature dependence) between the collector-emitter leakage current ICES [μA], which is the leakage current when the IGBT having a typical withstand voltage of 1,200 [V], used in an industrial inverter, and a junction temperature Tj [° C.].
FIG. 3 is a graph obtained by measuring the leakage current in a parallel connection in which the three collectors and the three emitters of the upper arm of the IGBT are connected by supposing a case where the IGBT is used in a three-phase inverter. The graph obtained by measurement by similarly parallelly connecting the three collectors and the three emitters of the lower arm of the IGBT perfectly agree with the graph of the upper arm, and therefore, in FIG. 3, only one graph is illustrated.
A value obtained by dividing a voltage of 1,200 [V] applied between the collector and emitter at the time of measurement by the leakage current ICES [μA] that flows from the collector to emitter, which is read from the graph in FIG. 3, is the equivalent insulation resistance value between the collector and emitter at each temperature of the IGBT. Based on the graph in FIG. 3, to which extent the leakage current of the IGBT affects the insulation resistance measurement of the conventional technique at each temperature is explained below.
At room temperature (25 [° C.], the leakage current when the IGBT is OFF is as small as about 0.3 [μA] and this corresponds to about 4 [GΩ] in terms of the equivalent insulation resistance value of the IGBT. This value is a sufficiently large value compared to the insulation resistance value (100 [MΩ] to 1 [MΩ]) of the motor to be measured, and therefore, it can be considered that the leakage current of the IGBT does not considerably affect the measurement accuracy of the insulation resistance of the motor at room temperature.
However, as the temperature of the IGBT rises, the leakage current of the IGBT increases exponentially. In the case where the conjunction temperature T is 80 [° C.], the leakage current of the IGBT is about 40 [μA] and this means that in terms of the equivalent insulation resistance value of the IGBT, the value reduces to about 30 [MΩ]. In this case, it can be concluded that the equivalent insulation resistance reduces to a level that affects the measurement accuracy due to the leakage current of the IGBT when measuring the insulation resistance value of the motor by the conventional technique.
Further, when the conjunction temperature Tj rises up to 100 [° C.], the leakage current when the IGBT is OFF increases to about 200 [μA] and in terms of the equivalent insulation resistance value of the IGBT, the value is about 6 [MΩ]. In this case, the equivalent insulation resistance value reduces to a resistance value equal to or less than the insulation resistance value of the motor to be measured, and therefore, it will become difficult to perform insulation resistance measurement with substantially a high accuracy.
As explained above, in the case where the IGBT having the characteristics as illustrated in FIG. 3 is used, the temperature range in which it is possible to detect insulation deterioration of a motor with a high accuracy by the conventional technique is limited the range of temperatures in the vicinity of room temperature or lower than room temperature and it is known that in the state where the temperature is high (e.g., immediately after running a motor by an inverter etc.), such a problem will occur that the accuracy in the insulation resistance measurement of a motor and in the detection of insulation deterioration is degraded considerably because of the influence of the leakage current of the semiconductor switching elements.
As explained above, the leakage current flowing through the semiconductor switching elements of the inverter connected to both the motor winding (coil) and the DC link unit overlaps the measurement current, and therefore, particularly at the time of high temperatures when the leakage current of the semiconductor switching elements increases, there has been such a problem that the accuracy in the insulation resistance measurement of a motor reduces because of the influence of the leakage current of the semiconductor switching elements.
The present invention has been made in view of these problems and an object of the present invention is to provide a motor drive device and an insulation resistance detection method of a motor that implement exact measurement of the insulation resistance value and the insulation deterioration detection of a motor with a simple configuration by securely eliminating the influence of the leakage current flowing through semiconductor switching elements included in an inverter also when the temperature is high while using a high voltage charged across a smoothing capacitor of a DC link unit originally included in the inverter as a power source.